Instrument manifolds are commonly utilized in differential pressure systems between the source of the differential pressure and the pressure transducer, monitor, or meter. In a typical installation, a three-valve instrument manifold is installed between an orifice flange and a transmitter, and is used to (a) normally transmit two different pressures to the transmitter, and (b) intermittently test the reliability of the transmitter. The testing of the transmitter may be accomplished by several techniques, including opening a "crossover valve" so as to subject the transmitter to the same pressure on both sides of the differential pressure transmitter.
Without regard to the number of control valves, prior art instrument manifolds are basically of two types: (1) those designed for direct coupling on the input and/or the output ends of the manifold; and (2) those designed for remote coupling. The manifold ends designed for direct coupling utilize a flange integral with the manifold body, while ports in the manifold ends designed for remote coupling are tapped for receiving threaded fittings.
The interconnection of an instrument manifold to both the orifice flange and the differential pressure sensor may thus be made by either a remote coupling or a direct (close) coupling. Referring first to a remote coupling for an orifice flange/manifold interconnection, this interconnection is typically made between the two threaded ports of the orifice flange and the two threaded input ports of the manifold by flaired-end pliable tubing and hydraulic end connectors. With this remote coupling, the manifold may be located at a selected distance generally exceeding six inches from the orifice flange, and the manifold is typically structurally supported separate from the orifice flange. Similarly, a remote coupling between the manifold and the transmitter may be made with pliable tubing and end connectors, and the transmitter may also be located a selected distance from the manifold and structurally supported separate from the manifold. An advantage for remote coupling relates to the flexibility in placing the instrument manifold and the pressure sensor at any desired location relative to the orifice flange. Also, remote coupling of manifold flanges has often been preferred because of the availability of instrument manifolds at reasonable costs having tapped 1/2 inch NPT input and output ports.
On the other hand, there are significant and sometimes critical advantages to direct coupling over remote coupling. Using direct coupling, the spacing between the transmitter and the orifice flange may be minimized to achieve a high speed of response to a change of differential pressure. This reduction in spacing also minimizes the detrimental effects on signal accuracy due to pressure pulsations in the flow lines between the orifice and the transmitter. Fewer fluid-tight interconnections are required for direct coupling so that there is a reduced number of leak points and increased pressure signal reliability. Each of the passageways interconnecting the orifice flange to the transmitter may be provided along a central axis, thereby simplifying rod-out operations and reducing maintenance costs. Also, installation costs may be substantially reduced when using direct coupling, in part because the instrument manifold and transmitter do not require separate support structures. The manifold and transmitter may be mounted on a single support, or both the manifold and transmitter may be sufficiently supported by their interconnections to the orifice flange affixed to the flow lines so as to require no additional support.
The disadvantages of the prior art are overcome by the present invention. Improved apparatus is hereinafter provided for securing a flange to a standard instrument manifold designed for remote coupling. Utilizing another orifice fitting flange, the valve manifold of the present invention can thus be easily mounted to the orifice fitting, and both the manifold and transmitter may be supported by the orifice fitting.